394 HEMICHORDA OR ENTEROPNEUSTA. 



(2) The body cavity is formed from segmentally arranged ccelom 



pouches ; but there is a pair of pre-oral pouches absent in 

 Annelids, and the segmental arrangement in the organs of 

 the body in Balanoglossus is, to say the least, very vague. 



(3) The heart lies, as in some Annelids, dorsal to the gut, not 



ventral as in Vertebrates; the dorsal vessel carries blood 

 forwards, the ventral backwards, as is usual in Annelids. 

 But the double nervous system is essentially different from 

 that of Annelids ; and the gill-slits are also, so far as we 

 know, unrepresented there. If there be a relationship 

 between Enteropneusta and Annelids, it must be a very 

 distant one, perhaps restricted to origin from some common 

 stock. 

 Besides these affinities, others have been ingeniously detected. Those 

 alleged to exist between Enteropneusta and Nemerteans, e.g. the exter- 

 nal ciliation, the unsegmented musculature, the correspondence of the 

 "notochord" and the Nemertean proboscis, the nature of the nervous 

 system, the sacculations of the gut, the arrangement of the gonads, are 

 perhaps even more unsatisfactory than those above cited. Again, the 

 resemblance between the Tornaria larva and that of Echinoderms is 

 unsatisfactory, because of our relative ignorance of the development of 

 the Tornaria. 



Here, then, we have a lesson in uncertainties, for all that we can say 

 is, that the Enteropneusta seem to be synthetic, possibly transitional 

 types, exhibiting affinities with various others, but differing markedly 

 from all. 



Appendix (1) to Enteropneusta. Cephalodiscus. 



A single species (Cephalodiscus dodecalophus) was dredged by the 

 Challenger in the Magellan Straits. It was at first described by 

 M'Intosh as a divergent Polyzoon, but the researches of Harmer point 

 to relationship with Balanoglossus. 



The minute stalked individuals occur associated together in a gelatin- 

 ous investment ; the colony may attain a size of 9 in. by 6 in. 

 The gut is curved, the anus being beside the mouth, beneath which are 

 two rows of ciliated hollow tentacles. These two characters, formerly 

 supposed to indicate Polyzoan affinities, may perhaps be adaptations to 

 the sedentary life. With Balanoglossus this type has been compared, on 

 account of the possession of the following characters : — (a) The body is 

 divided into three regions, which correspond to the proboscis, collar, and 

 trunk of Balanoglossus ; this is especially obvious in the young bud ; 

 (i) each of the three regions contains a ccelomic cavity, the most 

 anterior being single, while the other two are divided by a median par- 

 tition ; (c) the anterior pre-oral cavity opens to the exterior by two 

 pores (cf. proboscis pore of Balanoglossus) ; (d) the collar legion is also 

 furnished with two collar pores, which open beneath a fold or operculum 

 developed from the collar ; («) in the collar region the dorsal nervous 

 system is also placed, and is continued to some extent into the proboscis ; 

 (/) beneath the nervous system lies a diverticulum from the gut, which 

 extends towards the proboscis region ; this has been compared to the 



